One very basic aspect of face-to-face human communication is the ability of the two parties to both talk and be heard at the same time. This aspect of personal communication is quite important, insofar as it enables parties to a conversation to interrupt each other when necessary and thus build substantial efficiency into their communication. It is also important in other respects, for example, in the event that one party is beginning to speak about something which may, perhaps unknown to him, be uncomfortable to a party to the conversation, off the subject, incorrect or otherwise destructive of the object of the communication or wasteful of time.
While the ability to interrupt the speech of another seems quite natural, it is an aspect of face-to-face communication not found in many electronic telecommunications systems. Indeed, the ability to both speak and be heard at the same time presents technical complications in most telecommunication systems. For example, if we consider radio frequency carrier wave transmissions, if two parties to a conversation transmit at the same time, the signals will interfere with each other causing beat frequency oscillations, feedback and the like. Systems which solve this problem and thus allow the parties to the conversation to both speak and be heard at the same time are referred to as duplex systems. This can be achieved, for example, in the case of radio frequency communication, by a pair of transmitters operating with a respective pair of receivers at two different frequencies, one assigned to each of the transmissions of the parties.
In contrast, until the introduction of speaker telephones, virtually all telephones were duplex systems. Generally, the operative heart of such telephone systems during this period substantially comprised the equivalent of the series combination of a pair of variable resistance carbon microphones and a pair of electromagnetic earphones. As one caller spoke over the line, a diaphragm coupled to a carbon powder compartment in the microphone caused successive compressions to be exerted against the carbon powder in the compartment, thus varying the electrical resistance of the compartment. This in turn varied the current passing through the series circuit resulting in a modulation of the current passing through the series combination to generate the telephone signal. This modulated signal, modulated at the actual frequency of the voice signal being transmitted, was present in the telephone system at the telephone set of both participants to the telephone conversation. This system, which continues in use, in principle unchanged from the original instruments developed by Bell in the 1870's, as noted above, inherently has a duplex characteristic. Duplex communication is achieved because the audio frequencies involved do not cause unacceptable interference with each other and because the gain of the feedback loop between the carbon microphone and the earphone is less than one.
With the advent of speaker telephones, it became necessary to introduce into the telephone instrument an audio amplifier for receiving audio signals from the telephone central office and amplifying them to drive a loudspeaker. This immediately presented the problem of preventing feedback between a microphone adjusted for sensitivity to the voice of a person, who is not speaking directly and proximately into it, while making the system unresponsive to audio signals introduced into the environment by the loudspeaker which could be very close to the microphone. To somewhat better understand this problem, it must be kept in mind that the telephone is a two-wire system used to carry both the transmitted and received signal. If the transmitted signal is thus allowed to be amplified by the amplifier which amplifies the received signal which is also carried on the same two wires, ambient noise will be amplified and feedback oscillations are likely to ensue at normal levels of speaker amplitude.
One approach to this problem was embodied in speaker telephone systems which included separate microphones and loudspeakers, both of which had some directional characteristic designed to ensure that audio on the loudspeaker would be loud enough for the telephone user to hear while at the same time having, perhaps, less audio field strength at some point where the microphone was placed. In addition, the solution involved a microphone whose directional sensitivity characteristic was directed toward the mouth of the individual using the system with minimal sensitivity in the vicinity of the speaker.
Thus, design objectives involved reducing the gain of the feedback loop between the microphone and the speaker to less than one with the volume control for the system set at a level which would allow easy intelligibility of the signal.
Such an approach does not, in principle, provide a commercially acceptable level of performance, as, for example, it imposes limits on the location of the parties to the conversation, maximum loudspeaker volume and minimum user voice levels. Moreover, the provision of several microphones is required in order to achieve good spatial separation between the microphone and the speaker and, as a result, the system becomes somewhat cumbersome physically. As a practical matter, it is also necessary for the user to adjust the position of the various parts of the system as well as the volume on it. For persons without technical ability and patience, successful operation of such a system was a hit or miss proposition and, in practice, even a reasonable facsimile of the best possible performance of the system was seldom achieved, with most users settling for barely operational configurations.
Another approach to this problem and one which is probably most widespread in modern communication systems is the sacrifice of duplex operation to relatively trouble-free speaker telephone operation. Generally, these systems incorporate an electronic switch which either turns off the speaker when the user is speaking or disables the microphone when the party at the other end of the telephone is speaking and monitors signal magnitudes when signals are being produced at both ends of the telephone conversation to determine whether the system will receive or transmit.
In accordance with so-called "hybrid" technology, a solution to the speaker telephone problem, without the above difficulty, has been approached. Generally, such systems operate by introducing a hybrid electronic circuit, which is meant to approximate the complex impedance of the telephone system, and which is used to produce a cancellation signal. This cancellation signal, when added to the signal on the telephone system (comprising both the transmitted and received signal) results in generating a third signal which includes only the received signal, which third signal is, in turn, sent to the amplifier and loudspeaker of the speaker telephone system.
Some idea of the complexity of the problem of generating an effective hybrid circuit can be appreciated when one considers the range of overall impedances which must be accommodated by the system. It has been reported that in one series of measurements from a single location involving calls to the same local exchange, calls to other local exchanges, calls to suburban exchanges and long distance calls to different area codes, those measurements in the range of .+-.1 standard deviation at 200 to 5000 Hz yielded results varying from 450 to 1700 ohms. Thus, the 600 ohm impedance routinely referred to in the literature as the impedance of a telephone line is, from a practical standpoint, of minimal importance to many applications.
Moreover, while the purely resistive component of the impedance of a telephone line is for a given telephone call relatively well behaved, the reactive component includes both capacitive and inductive components. This results in the net impedance varying with frequency both in magnitude (resistance plus reactance) and in sign (net inductive reactance or capacitive reactance).
The net inductive/capacitive component is commonly referred to as the imaginary portion of the impedance or the reactance while the resistive component of the impedance is referred to as the real component.
While such an approach would in theory appear to provide a perfect solution to duplex speaker telephone operation, as a practical matter, the approach suffers from several inadequacies. Firstly, telephone system line impedances vary greatly from system to system in different parts of the country and even from exchange to exchange within the same city. Thus, it becomes necessary for the system to be installed and a complex impedance adjusted to minimize feed through of the signal to be transmitted into the telephone speaker amplifier. Naturally, this represents a substantial expense insofar as it involves having a technician on site for installation of the system. The increase in cost is significant enough that, for the great majority of users, such systems are not, from an economic standpoint, a practical option.
Moreover, even after such a system is installed, experience has shown that the complex impedance of the telephone lines will vary from call to call and from time to time depending upon the lines being used by the central office switching system, environmental factors, and the like. Thus, the above on site adjusted systems, at best, represent only an approximation and, for that matter, an approximation of irregular quality depending upon the nature of the particular telephone system with which they are used.
In an attempt to address this problem, systems such as that described in U.S. Pat. No. 5,172,410 of Chace have been proposed. In such systems, a multiple tone measurement and adjustment sequence at the beginning of a telephone call is used. This is achieved by applying a plurality of tones of different frequency to the telephone line, measuring the complex received signal and then adjusting a synthesized RLC hybrid circuit to more closely correspond to the telephone line and then repeating that process, until an acceptable degree of conformity between the hybrid and the telephone line is achieved. The obvious cost of such a system is the presence of tones at the outset of the call and the fixed response of the system to what may be varying and even live characteristics.